The need to produce concrete mixtures with recycled materials is becoming more
important than ever before. Not only does using recycled materials in concrete
mixtures create landfill avoidance, but it decreases the depletion of virgin raw
materials. The basis for this research was to investigate the effects of using recycled
materials, in varying amounts, on the fresh and hardened concrete properties. This
research includes the design of concrete mixtures composed of varying amounts of
recycled material replacements. The recycled materials in this study consisted of
ground granulated blast furnace slag (GGBFS), recycled concrete (crushed hardened
concrete) and crushed waste glass. The GGBFS was used as a replacement for the
cement. The recycled concrete and waste glass were used to replace the coarse and
fine aggregates , respectively. The concrete mixtures designed ranged from a twenty
five percent replacement to one hundred percent replacement with recycled materials.
These mixtures were compared against a standard concrete mixture using cement and
virgin aggregates. For comparison purposes, all mixtures were held constant in
regards to water to cementitious ratio. The fresh concrete properties examined
included slump, air content and unit weight. The hardened properties examined
included compressive strength, rate of strength gain, freeze -thaw durability,
permeability, and alkali -silica reactivity potential.
A concrete mixture composed entirely of recycled materials was developed.
This concrete mixture developed substantial strength and durability and is comparable
to a normal strength concrete mixture in several aspects. This concrete made from
100% recycled materials was a very low permeable concrete with a compressive
strength of 4300 psi (29 .6 MPa). A concrete composed of 50% and 75 % recycled
materials that achieved strengths of nearly 7000 psi (48 MPa) and 6300 psi (43.4
MPa) respectively were also developed . The beneficial and negative effects of using
recycled aggregates and GGBFS in a concrete mixture were determined. The
deleterious expansions caused by the waste glass reacting with alkalis in the
cementitious paste (ASR) were also determined. It was found that GGBFS , when
used at replacement levels of 50%, eliminated these concerns when waste glass is
used at even 1 00% aggregate replacement levels.
The point at which replacement with recycled materials becomes detrimental
to the concrete mixture , in regards to strength, durability and workability was
determined. A replacement of 50% recycled materials was determined to be an
optimum replacement amount for concrete. The use of recycled materials was
determined to be a benefit with regards to strength and durability up to 50% when
compared with a normal concrete made from virgin materials. However, it was shown
that even a concrete with recycled materials in excess of 50% can be very beneficial
and comparable to a normal , regular strength concrete. Although freeze-thaw
durability's decreased for concretes made with recycled contents in excess of75%,
the permeability ' s of these mixtures are extremely low and when coupled with
substantial strength, these concretes would be suitable for use in many applications.